It would appear that Microsoft is conservative in regards to the amount of disk space and purposely overestimates the amount you'll need. Taken from a Microsoft TechNet Article about the Office 2010 System Requirements underneath What About Disk Space?:
Conservatism. We tend to be overly conservative when drafting hard disk requirements, and we round up to the nearest GB or 0.5 GB. For example, if we measure an application’s footprint to be 1.63 GB, our requirement will be 2.0 GB. If our measurement reads 1.99 GB, we’ll make the system requirement 2.5 GB. Our requirements are larger than the actual disk space usage of the software – and we intentionally oversize them just to be safe.